Each year, monarch butterflies fly 3500 kilometers from breeding grounds in Canada to an overwintering site in Mexico. How do they keep on course? By using the sun as a compass, say researchers who devised a new flight simulator to study monarchs as they flew segments of a virtual migratory journey.
On their annual epic voyages, monarchs rely heavily on wind and upwellings of warm air to move in the desired southwestern direction. Easily blown off course, they must often check their bearings. Conflicting reports have suggested that monarchs orient themselves using the sun or Earth's magnetic field--or possibly both.
The new study shows that monarchs use the relative position of the sun at a particular time of day to set their course. Behavioral biologist Henrik Mouritsen of the University of Oldenburg in Germany and neurobiologist Barrie Frost of Queen's University in Kingston, Ontario, tethered butterflies in a specially crafted flight simulator that uses airflow from underneath to spur hours of flying without influencing which way the butterflies headed. The monarchs reliably chose a southwesterly path. But experimentally jet-lagged butterflies, whose day-night cycles had been shifted 6 hours in either direction, were duped into setting a course 90 degrees from normal, indicating that they steer according to where they think the sun should be at a given time of day. Additionally, the butterflies didn't respond to changes in a magnetic field, which has been proposed as a potential navigational trick for cloudy days. The researchers suspect, based on evidence from another study, that monarchs instead use patterns of polarized light visible through the clouds. Mouritsen and Frost report their findings in the 9 July online issue of the Proceedings of the National Academy of Sciences.
The flight simulator opens the door to studies of insect migration that are hard to tackle under field conditions, says biologist Evandro de Gama Oliveira of the Institute of Biological Sciences in Belo Horizonte, Brazil: "I think their apparatus is fabulous!" While echoing Oliveira's enthusiasm for the simulator, David Gibo, a behavioral ecologist at the University of Toronto in Mississauga, Ontario, cautions that the simulator does not mimic the aerodynamic forces butterflies experience during migratory flight. Air rising from below might give butterflies a sense that they are falling, inducing "emergency flight," he says. That might desensitize monarchs to subtle magnetic cues. Although Gibo doesn't question the importance of the sun compass, he says a secondary magnetic compass has not been ruled out.